This invention relates to devices for sensing the position of robot fingers, and in particular, relates to apparatus for providing linear sensor response to robot finger position.
The term "robot" is used to denote an electromechanical manipulator. The use of robots in industry has been growing at an accelerating pace in recent years. Robotic applications have also become more complex in nature, requiring closed loop feedback systems with integral sensing. Such feedback loops are utilized to evaluate characteristics of the objects being manipulated by the robot, and to provide the capability of real-time adjustments in the robotic process.
It is therefore extremely important to have adequate sensing apparatus incorporated in end-of-arm tooling of a robotic device to determine the type of workpiece being processed, whether the workpiece has been gripped correctly, and whether the workpiece is being handled or positioned correctly. Additionally, the successful use of robotic devices depends in large part upon the adaptability of such devices to accommodate different parts. Various sensors, such as optical and tactile sensors, are employed to generate necessary feedback signals.
In technologies applied to assembly line settings, for example, tactile sensors such as path, force or moment sensors are required to identify the position and size of parts to be manipulated.
In many cases, the signals generated by such sensors are compared to specific size thresholds.
Most conventional industrial robots include a gripper assembly, a signal processing network, and a process controller, which receives, for example, position signals from the gripper assembly, and transmits to the gripper assembly control signals via a control loop.
Feedback for controlling the execution of robotic processes may be provided by position sensors for robot fingers.
Finger position sensors for industrial robots are known in the art. A gripper assembly incorporating such a sensor is disclosed in U.S. Pat. No. 4,509,783 to Ionescu. In the Ionescu patent, a gripper device is disclosed having a body formed with a piston chamber in which a piston is mounted. A pair of spaced fingers is connected to the body, and a crossbar and toggle links connect adjacent fingers, so that actuation of the piston effects the operation of the fingers between an opened and closed position. A first sensor device is disposed between the fingers to measure the lateral distance therebetween, and a second sensor is disposed between the body and a moveable cap supported thereon to measure displacement between the cap and the body.
Another gripper device incorporating a sensor is disclosed in U.S. Pat. No. 4,611,296 to Niedermayr. The Niedermayr patent discloses a robot manipulator having a sensor mounted on a moveable part of the robot which is connected to a control unit through a programmable sensor interface. The Programmable sensor interface includes microprocessor elements and memory elements. Each sensor has three signal ranges allocated thereto, and the signal ranges are stored in the memory element.
One widely used type of linear displacement sensor used in robotic applications is based on the Hall effect. The Hall effect is a superior sensing technology because it provides a virtually infinite cycle life. The simplest form of Hall effect technology is the Hall effect element, constructed from a thin planar sheet of conductive material with output connections oriented perpendicular to the direction of current flow. When the Hall effect element is subjected to a magnetic field, the element responds with an output voltage proportional to the applied magnetic field strength. The voltage output is typically on the order of millivolts and requires additional electronics to achieve useful voltage levels.
The combination of a Hall effect element and associated amplification electronics is called a Hall effect transducer. Such Hall effect transducers convert a magnetic field to an electrical signal. Linear displacement can be measured using a Hall effect transducer by inducing motion of a magnet relative to the sensor element. The voltage output of the Hall element will then be proportional to the flux density at the point where the transducer is located.
Prior Hall effect transducers for robotic devices have employed a unipolar head-on configuration for the sensing of linear displacement. The term head-on refers to the manner in which the magnet moves relative to the tranducer's reference point. In the head-on mode, the magnet's direction of movement is directly toward and away from the transducer, with the magnetic lines of flux passing through the transducer's reference point. The magnet and transducer are positioned so that one pole of the magnet will approach the sensing face of the transducer.
Such prior art linear displacement sensors have several associated disadvantages, the most serious of which is non-linear response to displacement. In the unipolar head-on mode, the relationship between output voltage and the distance between magnet and sensor may be modelled by a decaying exponential curve. The curve is exponential because as the sensor approaches the magnet, the magnetic field becomes exponentially stronger. That is, exponentially more lines of flux are cut by the sensor as distance decreases.
When utilized in a robotic position sensor, unipolar head-on Hall effect transducers accordingly generate non-linear responses to changes in position of the sensed target.
It is therefore an object of the invention to provide an improved manipulator position sensor.
It is a further object of the invention to provide an improved manipulator position sensor utilizing Hall effect sensing and having a linear response to changes in manipulator position.
It is also an object of the invention to provide sensing apparatus which can store a plurality of position set points for comparison with sensed manipulator position.